The coating of substrates, e.g., glass, by cathode sputtering in plasma processes is well known. The sputtering can be done conventionally or using a reactive gas. In the case of using a reactive gas for sputtering, it is called reactive sputtering. To that end, a current or voltage source produces a plasma, which removes material from a target. The removed material is then coated on the substrate, e.g., a glass substrate. If a reactive process is used, the target atoms can combine with gas atoms or molecules, depending on the desired coating.
In particular, if reactive processes are being used, arcs can occur in the plasma process. Such arcs can be detrimental to the plasma process and can even destroy the coating. Therefore, it is necessary to detect arcs quickly and reliably. Often, arcs are detected by monitoring the output voltage of the power supply. In some cases, if there is a rapid fall in the output voltage, an arc is detected. In some cases, the current can be monitored. If there is an instant rise in the output current, this is also indicative of an arc. In particular, output current and output voltage can be monitored and compared to a threshold value respectively. However, the output current and the output voltage may change during plasma processing for other reasons, e.g., without an arc occurring.
The power supply may be a current regulated and/or a voltage regulated and/or a power regulated power supply. Setpoint values for current, voltage or power are given from outside, for example by a user. The power supply is then regulated to achieve the setpoint values at the output. If, for example, the power supply is in the power regulation mode with a desired setpoint for the power, a maximum setpoint for the current, and a maximum setpoint for the voltage, it is difficult to determine a good threshold value for arc detection, because the absolute value for voltage or current are varying with plasma impedance. Hence, if a fixed threshold value is used to detect arcs, arc detection is not very reliable.